SOLUTION

Transistors are three-terminal semiconductor devices used to regulate current or to amplify an input signal into a greater output signal.

NPN and PNP are types of transistors.

NPN Transistor

For an NPN transistor, when the emitter-base is forward biased, holes from the base (p-type) start to flow to the emitter side (n-type) and electrons start to flow from the emitter to the base.

PNP Transistor

For a PNP transistor, when the emitter-base junction is forward biased, holes from the emitter start to flow to the base. The direction of the arrow also indicates the same.